I have built a small electrostatic panel that is interacting strangely with its driving amplifier - a 2x110 watt Onkyo M-5100. The panel is very efficient and responds favorably to being driven by an amplifier with a high output impedance (the frequency response flattens and the bass is considerably extended).

As the Onkyo is your typical voltage source, I have simulated a transconductance amplifier by adding a 47 ohm resistor in series with the step-up transformer primary. I presently have no way of knowing the sort of load the panel presents to the amplifier, but similar designs of comparable sizes have been quoted as having a reactance of 30K ohms in the lower registers down to 3k ohms in the treble, which translates to a range of roughly 200 ohms to 20 ohms on the primary side. This seems reasonable, given the degree to which the 47 ohm series resistance impacts performance.

At any rate, measuring the voltage swing of the amplifier into a 100k resistor, I get a healthy +/- 60 volts before clipping (it would only take about 42 volts to generate 110 watts into 8 ohms). With the panel connected, however, I start to see the amplifier flat-topping much earlier - around +/- 20 volts. Clearly, headroom shouldn't be an issue and the amp should be miles away from activating its protection circuits since the series resistor would limit current draw to 1.3 amps in a worst case scenario. Nothing runs warm, even at the point of clipping.

I am baffled by the early clipping of an amplifier that should have ample voltage and current reserves. Can anyone think of some parameter I may be ignoring?

Is this behaviour frequency dependent? When you say you load the amplifier with 100K do you mean 100k on the secondary of the transformer?

I would not be inclined to dismiss the safe operating area protection system from consideration. The SOAR protection tends to track the boundaries of the curve of Ic and Vce. Thus the thermal power limit is reached at high current and small voltage at one extreme and low current and high voltage at the other. When we add some reactance to the load we get a phase shift between E and I so the tracking action gets disturbed.

I agree that it is hard to imagine protection being an issue with 47 ohms in series with the load, but we need to blame something for this odd behaviour!

Thanks for your response, Keith. For the headroom test, the speaker and transformer were completely out of the circuit. The amplifier was directly connected to a 100k resistor and the voltage swing was measured with my scope.

The clipping is very likely frequency dependent, and my next test should be with actual test tones rather than music.

You must also include the transformer capacitance when you are calculating your projected impedance

More info would be very helpful such as your panel size,D/S or capacitance and transformation ratio and transformer capacitance in order to help you understand what kind of load is being presented to your amplifier.

Are you using the Antek transformers?

Typically I have found it harder to drive a smaller panel properly (although not impossible) due to its smaller surface area.

This is mainly due to the transformer parasitic capacitance being much much greater than the panels capacitance.

For instance my little panel should present an 8 ohm load to my amplifier in my setup.
But, it is more like less than a .5 ohm at 20khz due to the larger capacitance of the transformer.
In my situation I have two use 4 cores to reducw this capacitance in order to get any performance out of it when 1 or 2 should be all I need.

Second,
What is your crossover frequency?
This plays a big factor on whether or not you are saturating the cores.
This causes a tremendous load on the amplifier when this happens and will cause it to clip as it presents basically a short to the amplifier.

A capacitance load requires an amplifier to supply aprox. 4 to 6 times the current in order to maintain the same voltage as it does when it is driving a pure resistance load.
Being that ESL's has a rising Frequency Response of 6db an octave or 20db per decade with proper equalization this becomes much less of an issue.
It is common to use resistors for this but it would be much easier to use active equalization as it is also needed to keep the lower frequency's from saturating the transformer.

Third,
If your transformers resonate frequency is in the audio band this will also cause the amplifier to clip prematurely as well.

The M-5100 does not contain any SOA current clipping/limiting circuitry, only simple relay protection for over-current.
So any clipping behavior would have to be from voltage clipping.

Just a thought...do you happen to remember if your were measuring upstream or downstream of the 47ohm resistor? If down stream, you would be measuring the 20Vp across the transformer primary, with the remaining 40Vp dropped across the 47ohm resistor. As others already mentioned, the amount of voltage drop would be frequency dependent since the input impedance of the transformer varies with frequency.

Thanks for the really involved follow-ups! My daughter's first birthday festivities have rightly taken priority over amp measurements this weekend, but I will be sure to report back with my findings after I have considered everything here.

One initial observation - though this is basically a glorified tweeter panel, I have been running without any crossover. I am probably saturating the life out of the transformer with low frequencies it doesn't have any hope of reproducing under the best of circumstances. My first order of business will be to put in a line level high-pass and check for clipping. If I'm going to go that route anyway, I suspect a first-order low-pass filter with the right knee would combat the rising response of the speaker in much the same way as the series resistor without all the stress on the amplifier.

Yes, Very good as this is exactly where I am at, at the moment.
I just rehashed through all of my data and re-measured my transformer cores.
I am now ready to build some new small panels as well.

I didn't have any filtering on my first setup and I except the eq's on my mixer this time I will build some filters as well.
I am planning a segmented design as well and plan to use an active filter along with the resistors for the segments.
I am hoping that this will drastically reduce the power requirements of the resistors as well as widen the Horizontal dispersion.

Thank you all for the great suggestions. I was finally able to devote some time to my project last night and the solution was as simple as adding a proper line-level active filter. I was able to use the scope to audition different combinations of series resistor and crossover slope/cutoff to arrive at the optimal voicing for the panel. I suspect that the transformers were simply getting saturated by bass content well outside of the scope of what the panels could hope to reproduce. Once everything was stabilized, I was even able to swap out my fancy transformers for ones with core sizes about a tenth as large with no degradation of performance.